Abstract Background Primary care provision is important in the delivery of health care but many countries face primary care workforce challenges. Increasing demand, enlarged workloads, and current and anticipated physician shortages in many countries have led to the introduction of mid-level professionals, such as Physician Assistants (PAs). Objective: This systematic review aimed to appraise the evidence of the contribution of PAs within primary care, defined for this study as general practice, relevant to the UK or similar systems. Methods Medline, CINAHL, PsycINFO, BNI, SSCI and SCOPUS databases were searched from 1950 to 2010. Eligibility criteria: PAs with a recognised PA qualification, general practice/family medicine included and the findings relevant to it presented separately and an English language journal publication. Two reviewers independently identified relevant publications, assessed quality using Critical Appraisal Skills Programme tools and extracted findings. Findings were classified and synthesised narratively as factors related to structure, process or outcome of care. Results 2167 publications were identified, of which 49 met our inclusion criteria, with 46 from the United States of America (USA). Structure: approximately half of PAs are reported to work in primary care in the USA with good support and a willingness to employ amongst doctors. Process: the majority of PAs’ workload is the management of patients with acute presentations. PAs tend to see younger patients and a different caseload to doctors, and require supervision. Studies of costs provide mixed results. Outcomes: acceptability to patients and potential patients is consistently found to be high, and studies of appropriateness report positively. Overall the evidence was appraised as of weak to moderate quality, with little comparative data presented and little change in research questions over time. Limitations: identification of a broad range of studies examining ‘contribution’ made meta analysis or meta synthesis untenable. Conclusions The research evidence of the contribution of PAs to primary care was mixed and limited. However, the continued growth in employment of PAs in American primary care suggests that this professional group is judged to be of value by increasing numbers of employers. Further specific studies are needed to fill in the gaps in our knowledge about the effectiveness of PAs’ contribution to the international primary care workforce.